Always checkout master when it matches the advertised HEAD

This parallels the CGit behavior of always using refs/heads/master
when it matches the remote advertised HEAD commit.

Change-Id: I5a5cd1516b58d116e334056aba1ef7990697ec30

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/CloneCommandTest.java

@@ -236,4 +236,20 @@ public class CloneCommandTest extends RepositoryTestCase {
 			assertTrue(e.getMessage().contains(dirName));
 		}
 	}
+
+	@Test
+	public void testCloneRepositoryWithMultipleHeadBranches() throws Exception {
+		git.checkout().setName(Constants.MASTER).call();
+		git.branchCreate().setName("a").call();
+
+		File directory = createTempDirectory("testCloneRepositoryWithMultipleHeadBranches");
+		CloneCommand clone = Git.cloneRepository();
+		clone.setDirectory(directory);
+		clone.setURI("file://" + git.getRepository().getWorkTree().getPath());
+		Git git2 = clone.call();
+		addRepoToClose(git2.getRepository());
+		assertNotNull(git2);
+
+		assertEquals(Constants.MASTER, git2.getRepository().getBranch());
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/api/CloneCommand.java

@@ -229,6 +229,12 @@ public class CloneCommand extends TransportCommand<CloneCommand, Git> {
 		final Ref idHEAD = result.getAdvertisedRef(Constants.HEAD);
 		if (idHEAD == null)
 			return null;
+
+		Ref master = result.getAdvertisedRef(Constants.R_HEADS
+				+ Constants.MASTER);
+		if (master != null && master.getObjectId().equals(idHEAD.getObjectId()))
+			return master;
+
 		Ref foundBranch = null;
 		for (final Ref r : result.getAdvertisedRefs()) {
 			final String n = r.getName();